1. Field of the Invention
This invention relates to an optical memory such as optical card or an optical disk. In particular, this invention relates to an electroluminescent device that in one embodiment has an organic or inorganic electroluminescent material capable of being driven by either a positive or negative electric field, so that the device is capable of being used with either a forward or a reverse electrical current. Most particularly, the invention relates to optical ROM and WORM devices based on an electroluminescent material device capable of being driven by an alternating electric field. A material for a WORM medium is also disclosed.
2. Description of Related Art
Existing optical memory systems use two-dimensional data carriers with one or two information layers. Most of the previous technical solutions to optical data recording propose registering changes in reflected laser radiation intensity in local regions (pits) of an information layer. Such changes can be a consequence of interference effects in relief optical discs of the CD-ROM type, burning of holes in the metal film, dye bleaching, local melting of polycarbonate in widely-used CD-R systems, change of reflection coefficient in phase-change systems, etc.
Three-dimensional, i.e., multilayer, optical storage systems provide comparatively higher storage capacity. However, they impose specific limitations and requirements on the construction and features of recording media, the techniques for data recording and reading, and especially the depth of the recording media.
Existing optical media, such as CD-ROM and DVD-ROM disks, are normally read in reflection mode. However, for a multilayer storage medium to be read in reflection mode, every information layer must have an at least partially reflective coating. As a consequence, when a layer close to the bottom of the medium is read, both the reading beam and the reflected beam pass through many such coatings, thereby attenuating the reflected beam to an unacceptable extent. Also, because existing optical media are read with coherent radiation, both beams are subject to diffraction and interference distortions on pits and grooves of the information layers.
That is why multilayer fluorescent discs with fluorescent reading are preferable, as they are free of partly reflective coatings. Diffraction and interference distortions are also reduced because of the incoherent nature of fluorescent radiation, its longer wavelength in comparison to the reading laser wavelength, and the transparency and homogeneity (similar reflection coefficients of different layers) of the optical media upon the incident laser and the fluorescent radiations. Thus, multilayer fluorescent discs have some advantages in comparison to reflective discs.
Optical memory cards are ROM or WORM media having a credit card form. The cards are highly durable, easily carried in a user's pocket, and unaffected by electrostatic and magnetic fields or heat. Optical memory cards surpass all other card technologies in terms of data capacity.
Information retrieval in fluorescent optical memory systems, in particular, fluorescent ROM and WORM optical memory cards, is realized with the help of external light sources. Reading light is absorbed in a fluorescent material held in information pits and excites the fluorescence of the material.
In an unrelated field of endeavor, inorganic electroluminescence devices were discovered by Destriau in 1936. Destriau observed that when suitably prepared inorganic zinc sulfide phosphor powders activated with small additions of copper were suspended in an insulator and an intense alternating electric field (15 kV) was applied with capacitor-like electrodes, light emission resulted.
Electroluminescent research gained further impetus with the advent of transparent conducting electrodes based on tin oxide (SnO2) in the late 1940's. Typically, early devices were composed of two sheets of electrically conductive material serving as electrodes, one a thin conducting backing and the other a transparent conductive film, placed on opposite sides of a plastic or ceramic sheet impregnated with the inorganic phosphor, such as zinc sulfide doped with small amounts of copper or magnesium. A transparent glass sheet placed next to the transparent conductive sheet served as an outermost protective substrate. Application of an alternating voltage to the electrodes caused an electric field to be applied to the phosphor. Each time the field would change, radiation having a wavelength in the visible range was emitted.
Although a large amount of research and investigation was conducted on the alternating current electroluminescent devices, the devices never achieved practice application although they were originally highly touted as a room lighting sources. Unfortunately, at high brightness levels the AC electroluminescent devices exhibited a very short life, and after about 1963, most of the research into the AC electro-luminescence devices was severely curtailed.
The most recent efforts in this area have been directed to a molecular carbon (e.g., a form of carbon known as fullerene-60) system. The high voltage drive requirements, the associated high cost of drive circuitry, poor stability and lack of color capability have made these devices cost prohibitive.
Two other inorganic devices, 1) direct current (DC) inorganic semiconductor light emitting diodes (LEDs) and 2) fluorescent ion doped inorganic semiconductor thin film devices, trace their origins to the mid-fifties. Light emitting diodes based on forward biased inorganic semiconductor p-n junctions are limited to small area applications as a result of color, efficiency and cost limitations. The other inorganic devices, fluorescent ion-doped inorganic semiconductor thin film devices, require high operating voltages to accelerate electrons or holes to sufficient energies in order to excite or ionize the fluorescent ion centers. Such high operating voltages result in thin-film instability and failure of such devices.
Electroluminescent organic materials include both molecular and polymer forms These materials include light emitting polymeric polypyridines such a poly(p-pyridines), co-polymers such as poly(phenylenevinylene pyridylvinylene) and molecular light emitters such as 8-hyrooxyquinoline. Insulating materials include a wide variety of ceramics such as aluminum oxide and inorganic and organic materials such as polysilane, polymethylmethacyline, nylon, cimeraldine base (an insulating polyaniline polymer) and organic molecules such as 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4 oxdiazole. Electrodes can be fashioned from any suitable conductor including, but not limited to, a wide variety of conducting materials including 1) indium tin oxide, 2) metals such as gold, aluminum, calcium, silver, copper, and indium, 3) alloys such as magnesium-silver, 4) conducting fibers such as carbon fibers, and 5) conducting organic polymers such as conducting doped polyaniline and conducting doped polymole.
In typical applications where the device is used for lighting and display, at least one of the electrodes is fashioned from a transparent material such as indium tin oxide or a partially transparent material such as conducting doped polyaniline. The insulator between the light emitting material and the transparent or partially transparent electrode is also transparent or partially transparent and fabricated from an optically clear insulating polymer such as polymethylmethacrylate or a partially transparent insulating polymer such as the insulating emeraldine base form of polyaniline. Partially transparent electrodes and insulators can be used to advantage to filter or clip unwanted portions (frequencies) of light emitted from the organic light emitter.
For ease of manufacture and insulation purposes, it is preferable to form the device on a substrate which also serves to protect and typically insulate the device during use. Glass and clear insulating plastic are preferable when the device is used for lighting and display purposes. The AC driven symmetrical device is especially suitable for light emissions from both sides of the device in which case both insulators and electrodes are at least partially transparent as well as any insulating substrates that may be used with one or both electrodes.
In the last decade, there has been an emerging interest in direct current (DC) molecular and polymer electroluminescence devices. A variety of organic molecules and conjugated polymers, copolymers and mixtures have been found to exhibit electroluminescent properties. Light-emitting diodes incorporating these materials have demonstrated all of the necessary colors (red, green, and blue) needed for display applications. However, a need continues to exist to lower the device operating voltages and to increase their light-emitting (output) efficiency. Further improvements in charge injection and the balancing of charge transport are needed. Because of the asymmetry of the device configuration, efficient charge injection occurs only in one direction (forward DC bias). Under reverse bias, most of the devices either degrade quickly or show very poor performance. Although efforts have been made to improve the charge injection efficiency by the use of low work function electrodes such as calcium or magnesium and the use of an electron transporting material to improve negative charge (electron) injection, such devices continue to be operational in only one direction. Similarly, efforts have been made to improve charge injection efficiency by the use of high work function electrodes such as indium tin oxide (ITO) or gold and the use of hole transporting materials to improve positive charge (hole) injection. Such devices also continue to be operational in only one direction.